Today, the vast majority of hydrogen peroxide is produced by the well-known anthraquinone process. A survey of the anthraquinone process and its numerous modifications is given in G. Goor, J. Glenneberg, S. Jacobi: “Hydrogen Peroxide” Ullmann's Encyclopedia of Industrial Chemistry, Electronic Release, 6th ed. Wiley-VCH, Weinheim June 2000, page 14. Generally, the anthraquinone loop process comprises the following steps:
(a) Hydrogenation of a working solution comprising an organic solvent or mixture of organic solvents, and one or more active anthraquinone compounds;
(b) oxidation of the resulting hydrogenated working solution to form hydrogen peroxide;
(c) extraction of hydrogen peroxide with water;
(d) stabilizing of the extracted aqueous hydrogen peroxide solution;
(e) drying of the working solution after extraction; and
(f) regeneration and purification of the working solution.
For each of the above distinct process steps, the Ullmann reference discloses numerous different possibilities.
Crude hydrogen peroxide solutions or concentrated hydrogen peroxide solutions obtained from the anthraquinone process contain a plurality of compounds in addition to hydrogen peroxide in low concentrations. These compounds are either impurities or additives like stabilizers. The impurities are compounds that are extracted from the working solution into the aqueous phase. They are mainly ionic or polar species like carboxylic acids, alcohols, carbonyl compounds and amines. These impurities are therefore also found in commercial hydrogen peroxide solutions.
For example, hydroquinone solvents that are commonly used in the above described process are nitrogen containing compounds like amides and ureas (see Ullmann supra page 6). Particularly preferred are tetraalkyl ureas like tetrabutyl urea. The use of these solvents result in amine impurities like monoalkyl or dialkyl, especially monobutyl and dibutyl, amines in the final hydrogen peroxide solutions. For example, the commercial hydrogen peroxide solution HYPROX® available from Degussa AG contains up to 200 wppm mono- and dibutyl amine based on the weight of hydrogen peroxide.
Depending on the final use of the hydrogen peroxide solutions, it is also known to conduct additional purification steps in order to obtain the required specification for the respective use of the hydrogen peroxide solution.
For example, DE-A 100 26 363 discloses a purification process for aqueous hydrogen peroxide solutions, whereby the solutions are treated with an anion exchange resin, a nonionic absorbing resin having a specific structure, and a neutral absorbing resin also having a specific macroporous structure. The hydrogen peroxide solutions obtained in this way are substantially free of cationic, anionic and organic impurities. Therefore, the solutions are particularly useful in microelectronics applications.
Similarly U.S. Pat. No. 4,999,179 discloses a process for purification of hydrogen peroxide solutions that contain, after purification, each metal cation in an amount of less than 5 ppb, each anion in an amount of less than 10 ppb and organic impurities in an amount of not more than 5 ppm in terms of total organic carbon content.
The drawback of such methods is that the purification is extremely expensive and can therefore, for economic reasons, not be used for the preparation of chemical mass products like propylene oxide. Furthermore, such highly purified hydrogen peroxide solutions are substantially free of anionic components like phosphates and nitrates that are necessary for the stabilization of aqueous—especially highly concentrated—hydrogen peroxide solutions for safety reasons.
From EP-A 100 119, it is known that propene can be converted by hydrogen peroxide into propene oxide if a titanium-containing zeolite is used as catalyst.
Since then, many investigations with respect to the effect of the addition of basic, acidic and ionic compounds either during preparation of the titanium silicalite catalyst or their presence in the reaction mixture on the activity and selectivity of the catalysts have been published.
From EP-A 230 949, it is known to neutralize the titanium silicalite catalyst either prior to its use in an epoxidation reaction or in situ with strong bases thereby introducing large amounts of alkali metal or alkaline earth metal ions into the reaction mixture. Said neutralization resulted in an increase in activity and selectivity to the desired olefin oxide in a batch process.
The experiments in EP-A 757 043, however, show that in a continuous process the activity is considerably reduced if the catalyst is neutralized prior to or during the reaction. Therefore, it is suggested to treat the catalyst prior to or during the epoxidation reaction with a neutral or acidic salt. The experimental data in EP-A 757 043 confirm that by addition of neutral or acidic salts the selectivity is increased but the activity is less reduced compared to the addition of a base. But EP-A 757 043 only shows examples wherein the catalyst is treated with the salt prior to the reaction and the catalyst is used in slurry form. Additionally, the experiments were only run for 8 hours but nevertheless show a dramatic drop in catalyst activity only after 4 hours, which is by no means acceptable for an industrial process.
Similarly, EP-A 712 852 teaches that by performing an epoxidation process catalyzed by titanium silicalite in the presence of a non-basic salt the selectivity is increased. All the examples are run in batch operation mode with a stirred catalyst slurry for one hour. Although it can be confirmed that the presence of non-basic salts may have a positive influence on catalyst selectivity in a short term experiment, it was discovered that even if non-basic salts are present in a reaction mixture for a continuous epoxidation reaction the activity and selectivity drops dramatically over time. Thus, the teaching of EP-A 712 852 does not lead to a reaction system that can be economically employed in a continuous epoxidation process using hydrogen peroxide in the presence of a heterogeneous catalyst.
In WO 00/76989, the influence of ionic components in commercially available aqueous hydrogen peroxide solutions that are used in epoxidation reactions as described in the above prior art documents is discussed. Ionic components, especially phosphates and nitrates, are added to commercially available aqueous hydrogen peroxide solutions as stabilizers to reduce hazardous decomposition of hydrogen peroxide. Contrary to the disclosure in the above prior art documents, WO 00/76989 teaches that the presence of ionic components in the reaction mixture—even those that have been added as stabilizers to commercial hydrogen peroxide—is detrimental to the long term selectivity in a continuous titanium silicalite catalyzed epoxidation reaction and should therefore be reduced to a minimum.
Contrary to the above prior art documents, continuous reactions running up to 300 hours were conducted showing that if ionic components are present in an amount of more than 100 ppm the long term selectivity is reduced. To solve this problem, it is suggested to remove ionic components from hydrogen peroxide solutions prior to use in epoxidation reactions by means of ion exchangers. Moreover, WO 00/76989 teaches that ammonium compounds and ammonia should be avoided under any circumstances since these compounds may lead to undesired side products by oxirane ring opening reactions with the formed olefin oxide. Although the teaching in WO 00/76989 leads to some improvement in long term selectivity compared to the above art, this improvement is still insufficient for an industrial scale process. Furthermore, this improvement can only be achieved with the complicated and, both in terms of investment and process costs, economically undesirable additional process step of ion exchange. Last but not least, removal of stabilizing ions like phosphate and nitrate from the hydrogen peroxide solution makes the process more hazardous and additional measures have to be taken to ensure safety during the entire process.
Contradicting the teaching of WO 00/76989, WO 01/57012 discloses that the use of crude hydrogen peroxide solutions directly obtained from the anthraquinone process having large amounts of, for example, sodium, nitrate, phosphate, and organic impurities, is superior with respect to product selectivity compared to highly purified hydrogen peroxide solutions containing very low amounts of sodium, nitrate, and phosphate. The experiments, however, were only conducted for a few hours so that the long term activity and selectivity of the catalyst cannot be determined from that reference.
Again another approach is disclosed in WO 01/92242, wherein a titanium silicalite catalyzed process for epoxidation of olefins using crude hydrogen peroxide solutions in the presence of a compound having aminocarbonyl functionality in which the nitrogen bears at least one hydrogen atom is disclosed. The examples show a batch type process that is conducted up to a conversion of hydrogen peroxide of 85%. After two hours the reaction is terminated even if the conversion of 85% has not been reached. Although the experimental data show an improvement with respect to the reaction rate compared to compounds with aminocarbonyl functionality having no hydrogen atom bonded to the nitrogen atom, long term activity and selectivity of the catalyst in a continuous process is not determinable from the information in WO 01/92242.
DE-A 199 36 547 discloses a continuous titanium silicalite catalyzed process for epoxidation of olefins with hydrogen peroxide whereby the conversion is kept constant by increase of reaction temperature and adjusting the pH of the reaction mixture. In a long term experiment (1000 hours), it could be verified that by adjusting the pH the increase in temperature and the rate of increase could be reduced compared to an experiment without pH adjustment. But conversion and selectivity were the same, irrespective of whether the pH was adjusted or not.
Thus, the object of the present invention is to provide an aqueous hydrogen peroxide solution that can be economically produced, that can be safely handled, stored, and shipped, and that is suitable for the epoxidation of olefin in the presence of a heterogeneous catalyst and leads to improved long term activity and selectivity of the catalyst.